Phospholipid directed motility of surface-motile bacteria.

Myxococcus xanthus is a surface-motile bacterium that has adapted at least one chemosensory system to allow directed movement towards the slowly diffusible lipid phosphatidylethanolamine (PE). The Dif chemosensory pathway is remarkable because it has at least three inputs coupled to outputs that control extracellular matrix (ECM) production and lipid chemotaxis. The methyl-accepting chemotaxis protein, DifA, has two different sensor inputs that have been localized by mutagenesis. The Dif chemosensory pathway employs a novel protein that slows adaptation. Lipid chemotaxis may play important roles in the M. xanthus life cycle where prey-specific and development-specific attractants have been identified. Lipid chemotaxis may also be an important mechanism for locating nutrients by lung pathogens such as Pseudomonas aeruginosa.

FibA and PilA act cooperatively during fruiting body formation of Myxococcus xanthus.

The extracellular matrix (ECM) of Myxococcus xanthus is essential for social (S-) motility and fruiting body formation. An ECM-bound protein, FibA, is homologous to M4 zinc metalloproteases and is important for stimulation by a phosphatidylethanolamine (PE) chemoattractant and for formation of discrete aggregation foci. In this work, we demonstrate that a correlation exists between a reduced ability to respond to PE and the observed defects in fruiting body morphogenesis. Furthermore, the fibA aggregation defect is accentuated by the absence of either PilA, the structural subunit of type IV pili, or DifD, a chemosensory response regulator. The inability to form fruiting bodies is not due to a loss of S-motility, but rather the loss of PilA and pili as pilT fibA mutants form fruiting bodies. The FibA active site residue E342 is important for fruiting body morphogenesis in the absence of PilA. Mutants exhibiting defects in fruiting body morphogenesis also produce fewer viable spores. It is proposed that FibA and PilA act as extracellular sensors for developmental signals.

Cohesion-defective mutants of Myxococcus xanthus.

Cohesion of Myxococcus xanthus cells involves interaction of a cell surface cohesin with a component of the extracellular matrix. In this work, two previously isolated cohesion-defective (fbd) mutants were characterized. The fbdA and fbdB genes do not encode the cohesins but are necessary for their production. Both mutants produce type IV pili, suggesting that PilA is not a major cohesin.

The Dif chemosensory pathway is directly involved in phosphatidylethanolamine sensory transduction in Myxococcus xanthus.

Myxococcus xanthus cells glide on solid surfaces and are chemotactically stimulated by certain phosphatidylethanolamine species. The dif gene cluster consists of six genes, difABCDEG, five of which encode proteins homologous to known chemotaxis proteins. DifA and DifE are required for the biosynthesis of fibrils, an extracellular matrix comprised of polysaccharide and protein. Chemotactic stimulation by 1,2-O-Bis[11-(Z)-hexadecenoyl]-sn-glycero-3-phosphatidylethanolamine (16:1 PE) and dilauroyl PE (12:0 PE) requires fibrils. Although previous work has shown that difA and difE mutants are not stimulated by 12:0 PE, these results do not distinguish between a dependence on fibrils or a direct role in chemosensory transduction. Here we provide evidence that the Dif chemosensory pathway directly mediates PE sensory transduction. First, stimulation by and adaptation to 16:1 PE requires all of the dif genes, including difBDG, which are not essential for fibril biogenesis. Second, a specific residue within the first putative methylation domain of DifA is required for stimulation by 16:1 PE but not fibril biogenesis. Transmembrane signalling through a chimeric NarX-DifA chemoreceptor is required for fibril formation but not for stimulation by or adaptation to 16:1 PE. Third, difD and difE are required for stimulation by dioleoyl PE (18:1 PE) although the response does not require fibrils. Taken together these results argue that the Dif pathway mediates both matrix formation and lipid chemotaxis.

An extracellular matrix-associated zinc metalloprotease is required for dilauroyl phosphatidylethanolamine chemotactic excitation in Myxococcus xanthus.

An extracellular matrix connects bacteria that live in organized assemblages called biofilms. While the role of the matrix in the regulation of cell behavior has not been extensively examined in bacteria, we suggest that, like mammalian cells, the matrix facilitates cell-cell interactions involved with regulation of cohesion, motility, and sensory transduction. The extracellular matrix of the soil bacterium Myxococcus xanthus is essential for biofilm formation and fruiting body development. The matrix material is extruded as long, thin fibrils that mediate adhesion to surfaces, cohesion to other cells, and excitation by the chemoattractant dilauroyl phosphatidylethanolamine. We report the identification of a putative matrix-associated zinc metalloprotease called FibA (fibril protein A). Western blotting with FibA-specific monoclonal antibody 2105 suggests extensive proteolytic processing of FibA during assembly into fibrils, consistent with the autoprocessing observed with other members of the M4 metalloprotease family. Disruption of fibA had no obvious effect on the structure of the fibrils and did not inhibit cell cohesion, excitation by dioleoyl phosphatidylethanolamine, or activity of the A- or S-motility motors. However, the cells lost the ability to respond to dilauroyl phosphatidylethanolamine and to form well-spaced fruiting bodies, though substantial aggregation was observed. Chemotactic excitation of the fibA mutant was restored by incubation with purified wild-type fibrils. The results suggest that this metalloprotease is involved in sensory transduction.